Sirat Ibn Ishaq
“'Abdullah b. Ubayy b. Salul went to him when God had put them in his power and said, 'O Muhammad, deal kindly with my clients' (now they were allies of Khazraj), but the apostle put him off. He repeated the words, and the apostle turned away from him, whereupon he thrust his hand into the collar of the apostle's robe; the apostle was so angry that his face became almost black. He said, 'Confound you, let me go.' He answered, 'No, by God, I will not let you go until you deal kindly with my clients. Four hundred men without mail and three hundred mailed protected me from all mine enemies; would you cut them down in one morning? By God, I am a man who fears that circumstances may change.' The apostle said, 'You can have them.'”
Ibn Ishaq's Sirat Rasul Allah (trans. Guillaume), p. 546 — 'Abdullah b. Ubayy b. Salul interceding for the Banu Qaynuqa'. Not a hadith-collection citation; no sunnah.com URL applies.