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.