Hadith 619
It is narrated from Sayyiduna Abu Hurairah (may Allah be pleased with him) that the Noble Prophet (peace and blessings of Allah be upon him) would only perform Qunoot Nazilah when he wanted to supplicate for someone or invoke against someone. When the Prophet (peace be upon him) would say « سَمِعَ اللهُ لِمَنْ حَمِدَهُ », after saying « رَبَّنَا وَلَكَ الحَمْدُ », he would supplicate, "O Allah, grant (so-and-so) salvation." Then the entire hadith was narrated.
Hadith 620
It is narrated from Sayyiduna Anas (may Allah be pleased with him) that the Noble Prophet (peace and blessings be upon him) would not recite Qunoot (continuously and always), except when he wished to supplicate for good for a people or to invoke against a people (then he would recite Qunoot).