There are some people who claim it is not allowed to say: sayyidi (short: sidi) or sayyidina. Let’s see what we can read in two well known books of giant scholars of Islam on this:
1. al-Adab al-Mufrad by imam al-Bukhari
2. Kitab al-Adhkar by imam al-Nawawi
In al-Adab al-Mufrad (hadith nr. 210), Bab Hal yaqulu sayyidi? we can read the following hadith: Abu Hurayrah related that the Prophet of Allah (s.a.w.s.) said: “No one should say ‘my slave’ or ‘my slave girl’. No slave should say ‘my master (rabbi)’ or ‘my mistress’. Instead, say ‘my servant’ or ‘my servant woman’ en ‘my sir’ (sayyidi) or ‘my lady’ (sayyidati). All of you are the slaves of Allah and the only Lord you have is Allah! This hadith was also related by Abu Dawud and al-Nasa’i. In the following hadith (nr. 211) we read that the Sahabah called the Prophet (s.a.w.s.) sayyidina and he answered that Allah is “Al-Sayyid”
In Kitab al-Adhkar imam al-Nawawi relates several similar ahadith as the one above and he says there is nothing wrong with calling someone sayyidi. I refer to the Kitab Hifz ul-Lisan, Fasl fi lafzh al-sayyid.
For further reading on this topic and related issues, read here: http://seekersguidance.org/ans-blog/2011/05/31/is-it-proper-to-add-sayyidina-before-the-names-of-our-prophets-muhammad-and-ibrahim-in-the-tashahhud/