«كان إذا أراد أن يدعوعلى أحد أو يدعولأحد قنت بعد الركوع»
It was the practice of the Prophet (peace and blessings be upon him):
"When he (peace and blessings be upon him) intended to supplicate against someone or for someone, he would perform qunut after the bowing (ruku')."
Hadith Referenceصحیح الجامع الصغیر / حرف الكاف / 4655