Sunan an-Nasa'i, Volume 6, Book 51, Hadith 5755 (DarusSalam) ⓘ
'Ubaidullah bin Sa'eed said: "I heard Abu Usamah say: 'I never saw any man more assiduous in seeking knowledge than 'Abdullah bin Al-Mubarak, not in Ash-Sham, Egypt, Yemen or the Hijaz.'"
أَخْبَرَنَا عُبَيْدُ اللَّهِ بْنُ سَعِيدٍ، قَالَ سَمِعْتُ أَبَا أُسَامَةَ، يَقُولُ مَا رَأَيْتُ رَجُلاً أَطْلَبَ لِلْعِلْمِ مِنْ عَبْدِ اللَّهِ بْنِ الْمُبَارَكِ الشَّامَاتِ وَمِصْرَ وَالْيَمَنَ وَالْحِجَازَ .
also referenced under: Hadith 5752 (Reference) & Book 51, Hadith 214 (In-Book)