كتاب السلم
Sales in which a Price is paid for Goods to be Delivered Later (As-Salam)
Chapter: As-Salam for a known specified weight
باب السَّلَمِ فِي وَزْنٍ مَعْلُومٍ
Sahih al-Bukhari 2240b

Narrated Ibn Abi Najih:

as above, saying, "He should pay the price in advance for a specified measure and for a specified period."

حَدَّثَنَا عَلِيٌّ، حَدَّثَنَا سُفْيَانُ، قَالَ حَدَّثَنِي ابْنُ أَبِي نَجِيحٍ، وَقَالَ، ‏ "‏ فَلْيُسْلِفْ فِي كَيْلٍ مَعْلُومٍ إِلَى أَجَلٍ مَعْلُومٍ ‏"‏‏.‏
Reference : Sahih al-Bukhari 2240b
In-book reference : Book 35, Hadith 4
USC-MSA web (English) reference : Vol. 3, Book 35, Hadith 444
  (deprecated numbering scheme)