Mishkat al-Masabih #1281
Prayer
The Arabic includes the chain of narration (sanad) and the collector's commentary; the English translation shows the Prophet's words only.
وَعَنْ نَافِعٍ قَالَ: إِنَّ عَبْدَ اللَّهِ بْنَ عُمَرَ كَانَ يَرَى ابْنَهُ عُبَيْدَ اللَّهِ يَتَنَفَّلُ فِي السَّفَرِ فَلَا يُنْكِرُ عَلَيْهِ. رَوَاهُ مَالِكٌ
Nafi' said that ‘Abdallah b. ‘Umar used to see his son ‘Ubaidallah saying supererogatory prayers while on a journey, and did not express disapproval.
Malik transmitted it.
About Mishkat al-Masabih
This hadith is #1281 from Mishkat al-Masabih, in the Book of Prayer.