Musannaf Ibn Abi Shayba 3:5337

Abu Bakr narrated to us, he said: Ibn Numayr narrated to us, he said: Ubayd Allah ibn Umar narrated to us, from Nafi’, from Ibn Umar, that he used to not stand [in prayer] with the people during the month of Ramadan. He said: And Salim and al-Qasim used to not stand [in prayer] with the people.