Al-Kāfi - Volume 3 > Times Zakat Is Due
Hadith #2

2ـ مُحَمَّدُ بْنُ يَحْيَى عَنْ أَحْمَدَ بْنِ مُحَمَّدٍ رَفَعَهُ عَنْ أَبِي بَصِيرٍ عَنْ أَبِي عَبْدِ الله (عَلَيْهِ الْسَّلام) قَالَ قُلْتُ لَهُ هَلْ لِلزَّكَاةِ وَقْتٌ مَعْلُومٌ تُعْطَى فِيهِ فَقَالَ إِنَّ ذَلِكَ لَيَخْتَلِفُ فِي إِصَابَةِ الرَّجُلِ الْمَالَ وَأَمَّا الْفِطْرَةُ فَإِنَّهَا مَعْلُومَةٌ.

2. Muhammad ibn Yahya has narrated from Ahmad ibn Muhammad in a marfu‘ manner from abu Basir who has said the following: “I once asked abu ‘Abd Allah, ‘Alayhi al-Salam, Is there a definite time to pay Zakat? He (the Imam) said, ‘That depends on one’s earnings. However, the time for al-Fitrah is definite.”’