Is “Seek for your brother seventy excuses” a Prophetic narration, and if so what is the full text of the narration and its chain of narrators?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.

This is not a Prophetic narration; rather it is the statement of Ja’far Ibn Ahmad. Al-Bayhaqi reported that Ja’far Ibn Ahmad said: 'If you hear anything you do not like about your brother, then seek one excuse for him to seventy excuses, if you find any then all perfect praise be to Allaah, otherwise say 'It might be that he has an excuse which I do not know about.'  Ibn ‘Asaakir reported this from Muhammad Ibn Sireen whose wording [Ibn Sireen’s] is: 'If you hear anything about your brother, then seek an excuse for him, if you do not find any then say 'It might be that he has an excuse.'

Allaah Knows best.