Selamun Aleikum,
In surah of Baqara, Ayat 15, begin with (Allah-u yestehziu)...
In yestehzi_u there is ya-i saqineh after za, before of ya-i sakineh there is kesrah (za), after that hemzah...
About Qiraat-i Hafs it must be medd-i muttasil. Why za isn't medd?
Answer
Wa alaikum assalaam wa rahmatullahi wa barakaatuh
In the phrase
, the word 
has a hamzah which uses the letter 
as a chair, meaning it is written on the , but the letter is not pronounced, instead only there for the hamzah to sit on. The therefore is not saakinah, it is not pronounced at all. This is why there is not medd here, as there is no saakinah. Instead the is just used as a placeholder for the hamzah. Wa assalaam