佛祖統紀 3.126

3.126

述曰。涅槃經云。佛告文殊。吾弟阿難。給事我來二十餘年。又後分經云。阿難偈曰。我為侍者二十載。故知此年始為侍者。

The commentary states: The Nirvāṇa Sūtra says: 'The Buddha addressed Mañjuśrī: "My younger brother Ānanda has served me for more than twenty years."' Furthermore, the Latter Portion Sūtra says: 'Ānanda's verse states: "I have been the attendant for twenty years."' Hence we know that it was in this year that he first became the attendant.