Masruq reported: Abdullah ibn Mas’ud, may Allah be pleased with him, said, “O people, fear Allah! Whoever among you knows something, let him say what he knows. Whoever does not know, let him say: Allah knows best. Verily, the most knowledgeable of you is one who says when he does not know: Allah knows best.”
Source: Ṣaḥīḥ Muslim 2798
Grade: Sahih (authentic) according to Muslim
عَنْ مَسْرُوقٍ قَالَ قَالَ عَبْدُ اللَّهِ بن مسعود رضي الله عنه يَا أَيُّهَا النَّاسُ اتَّقُوا اللهَ مَنْ عَلِمَ مِنْكُمْ شَيْئًا فَلْيَقُلْ بِمَا يَعْلَمُ وَمَنْ لَمْ يَعْلَمْ فَلْيَقُلِ اللهُ أَعْلَمُ فَإِنَّهُ أَعْلَمُ لِأَحَدِكُمْ أَنْ يَقُولَ لِمَا لَا يَعْلَمُ اللهُ أَعْلَمُ
2798 صحيح مسلم كتاب صفة القيامة والجنة والنار باب الدخان