Ikrimah (may Allah have mercy on him) says that Umar (may Allah be pleased with him) recited: "And let no scribe or witness be harmed" [Surah al-Baqarah: 282]. Sufyan says: A man comes to another and says: "Write for me." He says: "I am busy; find someone other than me." Then he must not harm him by saying: "I want no one but you"; rather, let him find someone else. And someone comes to a witness to make him witness something, and he says: "I am busy; take someone else." Then he must not harm him by saying: "I want only your testimony"; rather, let him take someone else as witness.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الشهادات / 20604