Mishkat al-Masabih 2 Knowledge كتاب العلم 50

وَعَنْ وَاثِلَةَ بْنِ الْأَسْقَعِ قَالَ: قَالَ رَسُولُ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ: «مَنْ طَلَبَ الْعِلْمَ فَأَدْرَكَهُ كَانَ لَهُ كِفْلَانِ مِنَ الْأَجْرِ فَإِنْ لَمْ يُدْرِكْهُ كَانَ لَهُ كفل من الْأجر» . رَوَاهُ الدِّرَامِي

Wathila b. al-Asqa‘ reported God’s messenger as saying, “He who seeks knowledge and attains it will have a double portion of reward, but if he does not attain it he will have a single portion of reward.” Darimi transmitted it.