Is it Sunnah to say Laa ilaaha ill-Allaah when parting from one’s friend?

Firstly: 

We do not know of any saheeh (sound) or da’eef (weak) hadeeth which speaks of this dhikr when parting or concluding a gathering. Hence persisting in it or believing it to be a dhikr that is prescribed on such occasions is a bid’ah that is be rejected, because the Prophet (peace and blessings of Allaah be upon him) said: “Whoever does any action that is not in accordance with our affair will have it rejected.” Narrated by Muslim (1718). 

The scholars have stated that singling out a particular time or place for an act of worship, or a particular manner in which it is to be done, with no proof from the texts, makes it a kind of bid’ah and innovation, in which case it is called bid’ah idaafiyyah. It is prescribed in principle, but is rejected because of the manner in which it is done. Acts of worship must be prescribed in and of themselves, the manner in which they are done, the time at which they are done and the number of times they are done, because Allaah can only be worshipped in the ways that He has prescribed in His Book or on the lips of the Messenger of Allaah (peace and blessings of Allaah be upon him). 

Al-Shaatibi (may Allaah have mercy on him) said: Bid’ah, therefore, refers to a fabricated religious way which competes with the way that is based on sharee’ah, whereby following it is intended to go to extremes in devotion and worship.

That also includes adhering to certain ways and forms of worship, such as reciting dhikr in unison, taking the day of the Prophet’s birthday as a festival, and so on. 

It also includes adhering to a specific act of worship at a specific time for which there is no basis in sharee’ah, such as always fasting on the fifteenth of Sha’baan and spending that night in prayer. 

End quote from al-I’tisaam (1/37-39). 

See also the answer to question no. 82559. 

Secondly: 

It is prescribed to end a gathering by saying that which was narrated by Abu Dawood (4859) from Abu Barzah al-Aslami (may Allaah be pleased with him) who said: The Messenger of Allaah (peace and blessings of Allaah be upon him) used to say, when he wanted to leave a gathering: “Subhaanaka Allaahumma wa bi hamdika, ashhadu an laa ilaaha illa anta, astaghfiruka wa atoobu ilayk (Glory and praise be to You, O Allaah, I bear witness that there is no god but You, I seek Your forgiveness and I repent to You).” And he said: “It is expiation for whatever happened in that gathering.” 

Classed as saheeh by al-Albaani in Saheeh Abi Dawood.  

It is also prescribed for one of the people present to recite Soorat al-‘Asr when ending the gathering, because of the report narrated by al-Tabaraani in al-Awsat (5124) from Abu Madeenah al-Daarimi (may Allaah be pleased with him) who was a companion of the Prophet (peace and blessings of Allaah be upon him). He said: When two men of the companions of the Prophet (peace and blessings of Allaah be upon him) met, they would not part until one of them recited “By Al‑‘Asr (the time), Verily, man is in loss” [al-‘Asr] over the other, then one would say salaam to the other. Classed as saheeh by al-Albaani in al-Silsilah al-Saheehah (no. 2648). 

Look at how people have turned away from a proven Sunnah for the sake of the bid’ahs that they have introduced, confirming the words of the Prophet (peace and blessings of Allaah be upon him): “No people introduce a bid’ah but something equivalent will be taken away from the Sunnah.” Narrated by Ahmad (16522). Al-Haafiz said in al-Fath (13/253): Its isnaad is jayyid. 

May Allaah enable us and you to follow the Sunnah and avoid bid’ah. 

And Allaah knows best.