Foot to foot or shoulder to shoulder?

The correct view is that in the rows of prayer, we should
stand shoulder to shoulder and foot to foot.

Al-Bukhaari, may Allaah have mercy on him, narrated (683)
from Anas that the Prophet
(peace and blessings of Allaah be upon him)
said: “Make your rows straight, for I can see you from behind my back.”

Anas said: “so each of us would stand with his shoulder
against his neighbour’s shoulder and his foot against his foot.”

Al-Bukhaari (may Allaah have mercy on him) gave this
chapter the title: “Chapter on standing shoulder to shoulder and foot to
foot.”

He said: al-Nu’maan ibn Basheer said: “I saw people
standing with their ankles against their neighbours’ ankle.”

Shaykh ‘Abd al-‘Azeem al-Abaadi said: he said in al-Ta’leeq
al-Mughni: These ahaadeeth clearly indicate the importance of making the
rows straight, which is part of the perfection of prayer; they indicate that
people should not stand back (from the row) or in front of (the row), and
that they should stand shoulder to shoulder, foot to foot and knee to knee
with their neighbours. But nowadays this Sunnah is being ignored! If someone
does this nowadays the people shy away from him like zebras! Verily to
Allaah we belong and unto Him is our return.

(‘Awn al-Ma’bood, 2/256)

The shoulder: is the place where the upper arm joins the
body.

The ankle: is the bone which protrudes at the side of the
foot.

And Allaah knows best.