Can someone who begins a voluntary fast break it?
It is preferable for someone who begins an act of worship not to break it.
Allah the Exalted has said {what means}: "and let not your [good] deeds come to nought!" [Muhammad/33].
However, if a person starts a voluntary fast (nafl) and needs to break it, they are going against what is preferable, but there is no sin upon them.
Does the time of Fajr (dawn) prayer start after the first Athaan (call for prayer), or the second one?
All perfect praise be to Allah,The Lord of The Worlds The time of Fajr prayer starts after the second Adhan. And Allah Knows Best.
What is the ruling on performing dry ablution (tayammum) on a wall or stone?
One of the conditions for tayammum is that it be with pure, clean dust (soil). It is not valid to perform tayammum by striking stones, a wall, sand that has no dust (i.e., that does not produce dust), pebbles alone, or clay, as these are not from the earth or its same category. And Allah the Almighty knows best.
What are the conditions governing the portion that must be distributed to the poor from a voluntary uḍḥiyyah (sacrificial offering)?
All praise is due to Allah, and may peace and blessings be upon our Master, the Messenger of Allah.
The following conditions apply to the portion distributed to the poor from the sacrificial animal:
First: It must be meat — it isn`t valid to give the poor organs or offal such as the liver.
Second: It must be given raw — cooked meat does not fulfil the requirement.
Third: It must amount to no less than half a kilogram. And Allah Almighty knows best.
