He thought that hastening to depart from Mina early meant leaving it even if one has not stoned the Jamaraat

This understanding of the word ta’ajjul (hastening) is not
correct, especially since for a pilgrim to “hasten” means that he leaves
Mina before sunset on the second of the days of al-tashreeq, after having
stoned the jamaraat. Simply leaving Mina before sunset does not count as
“hastening” if he has not stoned the jamaraat, especially if he comes back
after sunset to stone them. 

The scholars of the Standing Committee said: 

If the pilgrim stones the Jamaraat on the twelfth day after
the sun has passed its zenith, then departs to Makkah or elsewhere before
the sun sets, he does not have to stone the Jamaraat on the thirteenth, and
it is not prescribed for him to do so.

Fataawa al-Lajnah al-Daa’imah li’l-Buhooth al-‘Ilmiyyah
wa’l-Ifta’, 11/274. 

Commenting on the verse (interpretation of the meaning),
“And remember Allaah during the appointed Days. But whosoever hastens to
leave in two days, there is no sin on him and whosoever stays on, there is
no sin on him, if his aim is to do good and obey Allaah (fear Him)”
[al-Baqarah 2:203], the scholars of the Standing Committee said: 

With regard to the meaning of what is mentioned in the verse:
whoever hastens to depart from Mina after spending two nights there
following the Day of Sacrifice, and after stoning the three jamaraat on the
eleventh and twelfth, there is no sin on him and he does not have to offer a
(compensatory) sacrifice, because he has done what is required of him.
Whoever tarries in Mina and spends the night of the thirteenth there and
stones the three jamaraat on the thirteenth, there is no sin on him either,
rather his staying in Mina on this night and stoning the jamaraat on this
day is better and brings a greater reward, because the Prophet (peace
and blessings of Allaah be upon him) did that.

Fataawa al-Lajnah al-Daa’imah li’l-Buhooth al-‘Ilmiyyah
wa’l-Ifta’, 11/267. 

Shaykh Saalih al-Fawzaan said: 

The minimum length of
stay in Mina during the days of Hajj is the eleventh and twelfth – the two
days after Eid – and this is what is meant by hastening to leave. But it is
better to stay on the thirteenth too, which is what is meant by staying on.
Allaah says (interpretation of the meaning): 

“But whosoever hastens to leave in two days, there is no
sin on him and whosoever stays on, there is no sin on him, if his aim is to
do good and obey Allaah (fear Him)”

[al-Baqarah 2:203]

Hastening to leave means going away from Mina after stoning
the Jamaraat on the twelfth day after the sun has passed its zenith and
before the sun sets. If sunset comes and a person has not left Mina yet, he
is obliged to stay there on the night of the thirteenth and stone the
jamaraat after Zuhr on the thirteenth. And Allaah knows best. 

Fatwa no. 16386 

Shaykh Ibn ‘Uthaymeen was asked: 

A pilgrim did not stone the jamaraat on the twelfth because
he thought that this was part of hastening to leave Mina, and he did not
stay overnight in Mina or perform tawaaf al-wadaa’ out of ignorance. 

He replied: 

Your Hajj is valid, because you did not omit any pillar or
essential part of the Hajj, but you omitted three obligatory duties. The
first was staying overnight in Mina on the night of the thirteenth. The
second was stoning the jamaraat on the twelfth. The third was performing
tawaaf al-wadaa’ (the farewell tawaaf). According to the scholars, if a
person omits this tawaaf during Hajj, he has to offer a compensatory
sacrifice, to be slaughtered in Makkah and its meat distributed to the poor.
But not staying overnight in Mina for one night does not require a
compensatory sacrifice. 

I would like to take this opportunity to draw the attention
of my pilgrim brothers to this mistake made by our brother who asked the
question, because many pilgrims take the words of Allaah “But whosoever
hastens to leave in two days” to mean leaving Mina on the eleventh, so
they count the two days as the day of Eid and the eleventh, but that is not
how it is, rather this is a misunderstanding, because Allaah says
(interpretation of the meaning): 

“And remember Allaah during the appointed Days. But
whosoever hastens to leave in two days, there is no sin on him”

[al-Baqarah 2:203]

The appointed days (al-ayyaam al-ma’doodaat) are the
ayyaam al-tashreeq, the first of which is the eleventh. Based on this, the
words of Allaah “But whosoever hastens to leave in two days” mean two
of the days of al-tashreeq, i.e., on the twelfth. We have to make sure that
we understand such matters correctly so that we will not make mistakes. 

Fataawa al-Hajj, question no.
40. 

And Allaah knows best.