Allah’s سبحانه و تعالى instruction to Moosa عليه السلام when confronting Firaun:
فَقُولَا لَهُ قَوْلًا لَّيِّنًا لَّعَلَّهُ يَتَذَكَّرُ أَوْ يَخْشَىٰ
“And speak to him with gentle speech that perhaps he may be reminded or fear (Allah).” Quran 20:44
Allah سبحانه و تعالى tells Prophet Muhammad صلى الله عليه و سلم
فَبِمَا رَحْمَةٍ مِّنَ اللَّهِ لِنتَ لَهُمْ ۖ وَلَوْ كُنتَ فَظًّا غَلِيظَ الْقَلْبِ لَانفَضُّوا مِنْ حَوْلِكَ
“It was by a mercy from Allah that you were gentle with them. Had you been fierce and hard of heart, they would have scattered from round about you.” Quran 3:159
Imam al-Haddad says:
When you wish to give advice to someone regarding any behaviour of his that you have come to know about, be gentle, talk to him in private and do not express explicitly what may be conveyed implicitly. Should he ask you to tell him who told you that which you know, do not tell him lest it stir up enmity. If he accepts your advice, praise Allah, and thank Him. If he should refuse, blame yourself.