An Oleh is considered to be an Israeli citizen from their date of Aliyah.
An Oleh from a country which does not allow dual citizenship has a 90 day period following date of Aliyah during which they may decline Israeli citizenship.
How does a person decline Israeli citizenship?
Why would a person decline Israeli citizenship?
Generally because they are a citizen of a country where dual citizenship is not allowed.
Travelling abroad subsequent to Aliyah
An Oleh can enter and exit Israel using their foreign passport for 90 days following their date of Aliyah.
90 days after receiving Aliyah status, an Oleh is required to have a Teudat Ma'avar (Temporary Passport / Laissez Passer) in order to exit the country.