Sayyidah Aishah Siddiqah (may Allah be pleased with her) narrates: The Prophet (peace and blessings be upon him) used to recite: 'Allahumma ahsanta khalqi fa ahsin khuluqi' (O Allah, You have made my outward form beautiful, so make my character good as well).
Ahmad recorded it, and its narrators are the narrators of the Sahih.
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الأدعية / 17364