Narrated Abdullah ibn Amr ibn al-'As: I used to write everything which I heard from the Messenger of Allah (ﷺ). I intended (by it) to memorise it. The Quraysh prohibited me saying: Do you write everything that you hear from him while the Messenger of Allah (ﷺ) is a human being: he speaks in anger and pleasure? So I stopped writing, and mentioned it to the Messenger of Allah (ﷺ). He signalled with his finger to him mouth and said: Write, by Him in Whose hand my soul lies, only right comes out from it.
Source: sunnah.com
Other translations
Arabic
حَدَّثَنَا مُسَدَّدٌ، وَأَبُو بَكْرِ بْنُ أَبِي شَيْبَةَ قَالاَ حَدَّثَنَا يَحْيَى، عَنْ عُبَيْدِ اللَّهِ بْنِ الأَخْنَسِ، عَنِ الْوَلِيدِ بْنِ عَبْدِ اللَّهِ بْنِ أَبِي مُغِيثٍ، عَنْ يُوسُفَ بْنِ مَاهَكَ، عَنْ عَبْدِ اللَّهِ بْنِ عَمْرٍو، قَالَ كُنْتُ أَكْتُبُ كُلَّ شَىْءٍ أَسْمَعُهُ مِنْ رَسُولِ اللَّهِ صلى الله عليه وسلم أُرِيدُ حِفْظَهُ فَنَهَتْنِي قُرَيْشٌ وَقَالُوا أَتَكْتُبُ كُلَّ شَىْءٍ تَسْمَعُهُ وَرَسُولُ اللَّهِ صلى الله عليه وسلم بَشَرٌ يَتَكَلَّمُ فِي الْغَضَبِ وَالرِّضَا فَأَمْسَكْتُ عَنِ الْكِتَابِ فَذَكَرْتُ ذَلِكَ لِرَسُولِ اللَّهِ صلى الله عليه وسلم فَأَوْمَأَ بِأُصْبُعِهِ إِلَى فِيهِ فَقَالَ " اكْتُبْ فَوَالَّذِي نَفْسِي بِيَدِهِ مَا يَخْرُجُ مِنْهُ إِلاَّ حَقٌّ " .
sunnah.com