Mishkat al-Masabih 147
Rafi‘ b. Khadij said:
God’s Prophet came to Medina when they were fecundating the palm trees. He asked, “What are you doing?” and they replied, “We have been accustomed to do it.” He said, “Perhaps if you did not do it it would be better so they gave it up, but the crop was diminished. They mentioned that to him and he said, “I am only a human being. When I issue any command to you regarding your religion, accept it; but when I issue any command to you based on my own opinion, I am merely a human being.” 

Muslim transmitted it.
وَعَن رَافع بن خديج قَالَ: قَدِمَ نَبِيُّ اللَّهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ وهم يأبرون النَّخْلَ فَقَالَ: «مَا تَصْنَعُونَ» قَالُوا كُنَّا نَصْنَعُهُ قَالَ «لَعَلَّكُمْ لَوْ لَمْ تَفْعَلُوا كَانَ خَيْرًا» فَتَرَكُوهُ فنفضت قَالَ فَذَكَرُوا ذَلِكَ لَهُ فَقَالَ: «إِنَّمَا أَنَا بَشَرٌ إِذَا أَمَرْتُكُمْ بِشَيْءٍ مِنْ دِينِكُمْ فَخُذُوا بِهِ وَإِذَا أَمَرْتُكُمْ بِشَيْءٍ مِنْ رَأْي فَإِنَّمَا أَنا بشر» . رَوَاهُ مُسلم