Abu Qilaabah said:

May Allaah fight the poet who said: Do not ask about a man, but ask about his companion. For every person guides himself by his companion. This poetry is by Adee bin Zaid, and al-Asma'ee said about it, "I have never seen a line of poetry which resembles the Sunnah more than this saying of Adee bin Zaid. Al-Ibaanah (2/439).

Abu Hurairah - radiallaahu anhu - said:

The Messenger of Allaah said, "A person is upon the deen of his friend, so let each one of you look at whom he befriends." Saheeh. Refer to Silsilah as-Saheehah of al-Albaani (no.927).

Ibn Mas'ood - radiallaahu anhu - said:

Indeed a person walks alongside and accompanies the one whom he loves and who is like him. Al-Ibaanah (2/476).

Abu ad-Dardaa - radiallaahu anhu - said:

It is from the fiqh (understanding of a person) that he [chooses] those whom he walks with, whom he enters upon (visits) and whom he sits with. Al-Ibaanah (2/477).

Yahyaa bin Katheer said:

Sulaimaan bin Daawood - alaihis salaam - said: "Do no pass a judgement over anyone with anything until you see whom he befriends." Al-Ibaanah (2/464).

And Ibn Battah states:

Moosaa bin Uqbah the Syrian approached Baghdad and this was mentioned to Imaam Ahmad. So it was said, "Look at whose residence he goes to and with whom he resides and finds shelter." Al-Ibaanah (2/480).

Imaam Al-Awzaa'ee said:

Whoever hides his innovation from us will not be able to hide his companionship from us. Al-Ibaanah (2/476).

Al-A'mash said:

They (the Salaf) did not used to ask anything more about a person after having asked about three affairs: Who he walks with, who he enters upon (i.e. visits) and who he associates with amongst the people. Al-Ibaanah (2/478).

Abdullaah bin Mas'ood - radiallaahu anhu - said:

Make consideration of the (regions of the) Earth by the names given to them and make consideration of a person by his companion. Al-Ibaanah (2/479).

Muhammad bin Ubaid al-Ghulaabee said:

The Ahl ul-Ahwaa (People of Desires) hide everything except their intimate friendship and companionship. Al-Ibaanah (2/482).

Mu'aadh bin Mu'aadh said to Yahyaa bin Sa'eed:

O Abu Sa'eed! A person may hide his viewpoint from us, but he will not be able to hide that in his son, or his friend or in the one whom he sits with. Al-Ibaanah (2/437).

Ibn 'Awn said:

Those who sit with the People of Innovation are more severe upon us than the People of Innovation themselves. Al-Ibaanah (2/273).

Yahyaa bin Sa'eed al-Qattaan said:

When Sufyaan ath-Thawree came to Basrah he began to look into the affair of ar-Rabee' bin Subaih and the people's estimation of him. He asked them, 'What is his madhhab?', and they said, 'His madhhab is but the Sunnah'. He then asked, 'Who is his companionship?' and they replied, 'The people of Qadr' so he replied, 'In that case he is a Qadari'." Al-Ibaanah (2/453). Ibn Battah (after quoting this) said, "Allaah's mercy be upon Sufyaan ath-Thawree. He has indeed spoken with wisdom and he spoke the truth. He spoke with knowledge that is in agreement with the Book and the Sunnah and what is necessitated by wisdom and what the people of sure insight know. Allaah the Exalted said, "O you who believe! Take not as (your) Bitanah (advisors, consultants, protectors, helpers, friends, etc.) those other than you (outside your religion or upon other than the right way) since they will not fail to do their best to corrupt you. They desire to harm you severely." (Aali Imraan 3:118)" (Al-Ibaanah 2/453).

Abu Dawood as-Sijistaanee said:

I said to Abu Abdullaah Ahmad bin Hanbal: I see a man from Ahl us-Sunnah with a man from Ahl ul-Bid'ah. Shall I leave his speaking to him? He said, 'No, inform him that the man that you saw him with is a person of innovation, so if he leaves him, then speak to him, otherwise treat him the same as him, as Ibn Ma'sood said, 'A man is like his companion'. Tabaqaat ul-Hanaabilah (1/160).