Reference for Berakhot 49:17

ובודאן עד כמה אמר רב יהודה אמר שמואל כל זמן שמטפיחין וכן אמר רבה בב"ח א"ר יוחנן כל זמן שמטפיחין וכן אמר עולא כל זמן שמטפיחין גניבא משמיה דרב אמר כל זמן שרשומן ניכר

Where it is certain [that urine is present], how long [must one wait before it is allowed to read the Shema' there] ? Rab Judah said in the name of Samuel : As long as the ground is sufficiently wet to moisten. The same reply is given by Rabbah b. Bar Hannah in the name of R. Johanan [As long as the ground is sufficiently wet to moisten] and by 'Ulla. [As long as the ground is sufficiently wet to moisten] Geniba said in the name of Rab : As long as the trace thereof can be recognised.

Jastrow