Sunan an-Nasa'i 4451

أَخْبَرَنَا يُوسُفُ بْنُ عِيسَى، قَالَ أَنْبَأَنَا الْفَضْلُ بْنُ مُوسَى، قَالَ أَنْبَأَنَا الأَعْمَشُ، عَنْ إِبْرَاهِيمَ، عَنِ الأَسْوَدِ، عَنْ عَائِشَةَ، قَالَتْ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم ‏ "‏ إِنَّ أَطْيَبَ مَا أَكَلَ الرَّجُلُ مِنْ كَسْبِهِ وَوَلَدُهُ مِنْ كَسْبِهِ ‏"‏ ‏.‏

It was narrated that 'Aishah said:"The Messenger of Allah said: 'The best (most pure) food that a man eats is that which he has earned himself, and his child (and his child's wealth) is part of his earning

Reference

Sunan an-Nasa'i 4451

In-book

Book 44, Hadith 3