Fiqh-us-Sunnah Volume 3, Zakaat and Fasting, Fiqh 3.011.

Zakah must be paid immediately at its due time. Deferring payment of zakah is prohibited, unless the payer for some valid reason cannot pay it on time. In such a case, he may wait until he is able to pay it. It is related by Ahmad and al-Bukhari that ‘Uqbah ibn al-Harith said: “Once I performed the ‘asr prayer with the Prophet, upon whom be peace. When he concluded the prayer, he hurriedly went to his house and retuned immediately. Noticing the amazed faces, he said: ‘I left at home a piece of gold which was meant for sadaqah, and I did not want to let it remain a night in my house, so I ordered it to be distributed.'”

Ash-Shaf’i and al-Bukhari (the latter in his Tarikh) relate from ‘Aishah that the Prophet, upon whom be peace, said: “Whenever sadaqah which is payable is mixed with a property, it will destroy that property.” The same hadith is related by al-Humaydi with this addition: “If you have to pay sadaqah which is payable, then it must be set aside, or the unlawful [property] will destroy the lawful one.”