I've always seen it written as "tabee", but heard it mostly as "tabae".
Here are a few suggestions for finding out which is right:
-I would checkout out Giani Gian Singh's Tvareek Guru Khalas, since this Dohra is normally attributed to him paraphrasing what Guru Gobind Singh Ji said to Sikhs in Nanded. Another place that it has been reference.
-I also have heard people quote this from Rattan Singh Bhangu's Panth Prakash (speaking of which, it may be worth checking Giani Gian Singh's Panth Prakash as well), so that may be another place to look.
-Another place to look is Bhai Prehlad Singh's Rehatnama, which features a slightly different version (for example, it says "Guru Khalsa Maniyo Pragat Guraa Ki Deh, Jo Sikh Mo Milbey Chehe Khoj Inhu Mai Leho").
-Bhai Nand Laal Ji's Rehatnama includes Raj Karega Khalsa, but doesn't mention Agiya Bhaei Akaal Ki, so not much luck there.
-Maybe we can look at old Gutkey and see what is there (that is if the Dohra is included in them)
-There is a diary of Guru Gobind Singh that was written by Bhai Har Singh, which presides in Hazoor Sahib, may contain the Dohra there. That diary supposedly also contains the reference to "Nanak Naam Chardi Kala, Tere Bhaney Sarbat Da Bhala", so it may contain this Dohra.
-I haven't gone to Hazoor Sahib, but I've heard there is a written version of this Hukam by Guru Ji written there (and possibly on display).
Anyways, I personally say "tabee" because it makes a bit more sense to me given the knowledge and skills I have, but I could be wrong.
ਸਤਿਗੁਰਬਚਨਕਮਾਵਣੇਸਚਾਏਹੁਵੀਚਾਰੁ॥