Sayyiduna Ibn Abbas (may Allah be pleased with them both) said that the Messenger of Allah (peace and blessings be upon him) said: "'Allahumma in taghfir taghfir jamma, wa ayyu 'abdin laka la alamma' (O Allah, if You forgive, then forgive all; and which servant of Yours has not committed a sin?)"
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الشهادات / 20744