Musnad Ahmad 790:179

Muhammad ibn Idris—meaning al-Shafi‘i—narrated to us, Sufyan informed us, from ‘Ali ibn Zayd ibn Jud‘an, from al-Hasan, from ‘Imran ibn Husayn, that ‘Umar ibn al-Khattab said: "I adjure by Allah any man who heard anything from the Prophet, may Allah bless him and grant him peace, regarding the grandfather." A man stood up and said: "I witnessed the Prophet, may Allah bless him and grant him peace, give him a third." He said: "With whom?" He said: "I do not know." He said: "May you not know."