Translation
(35) Sales in which a Price is paid for Goods to be Delivered Later (As-Salam)
كتاب السلم
(1) Chapter: As-Salam by a definite known specified measure
(1) باب السَّلَمِ فِي كَيْلٍ مَعْلُومٍ
Sahih al-Bukhari 2239b 

Narrated Ibn Abi Najih:

as above, mentioning only specific measure.

حَدَّثَنَا مُحَمَّدٌ، أَخْبَرَنَا إِسْمَاعِيلُ، عَنِ ابْنِ أَبِي نَجِيحٍ، بِهَذَا ‏ "‏ فِي كَيْلٍ مَعْلُومٍ وَوَزْنٍ مَعْلُومٍ ‏"‏‏.‏
Reference : Sahih al-Bukhari 2239b
In-book reference : Book 35, Hadith 2
USC-MSA web (English) reference : Vol. 3, Book 35, Hadith 442
  (deprecated numbering scheme)