كتاب العلم
Chapter: The Prophet (saws) used to take care of the people in preaching by selecting a suitable time so that they might not run away (never made them averse or bored them with religious talk and knowledge all the time)
باب مَا كَانَ النَّبِيُّ صلى الله عليه وسلم يَتَخَوَّلُهُمْ بِالْمَوْعِظَةِ وَالْعِلْمِ كَىْ لاَ يَنْفِرُوا
Sahih al-Bukhari 69

Narrated Anas bin Malik:

The Prophet (ﷺ) said, "Facilitate things to people (concerning religious matters), and do not make it hard for them and give them good tidings and do not make them run away (from Islam).

حَدَّثَنَا مُحَمَّدُ بْنُ بَشَّارٍ، قَالَ حَدَّثَنَا يَحْيَى بْنُ سَعِيدٍ، قَالَ حَدَّثَنَا شُعْبَةُ، قَالَ حَدَّثَنِي أَبُو التَّيَّاحِ، عَنْ أَنَسٍ، عَنِ النَّبِيِّ صلى الله عليه وسلم قَالَ ‏ "‏ يَسِّرُوا وَلاَ تُعَسِّرُوا، وَبَشِّرُوا وَلاَ تُنَفِّرُوا ‏"‏‏.‏
Reference : Sahih al-Bukhari 69
In-book reference : Book 3, Hadith 11
USC-MSA web (English) reference : Vol. 1, Book 3, Hadith 69
  (deprecated numbering scheme)